The reduction in available system memory depends on the devices that are installed in the computer. However, to avoid potential driver compatibility issues, the 32-bit versions of Windows Vista limit the total available memory to 3.12 GB. See the "More information" section for information about potential driver compatibility issues.
From all of the PC's i've seen using 4GB of RAM on a Windows 32-bit OS, it varies anywhere from 3-3.2GB, although it's not always exact.I don't think 3.12 is an absolute maximum. http://support.microsoft.com/kb/929605 just uses that number as an example